In the arts, opus number usually denotes a work of musical composition, a practice and usage established in the seventeenth century when composers identified their works with an opus number. In the nineteenth century, publishers usually assigned opus numbers when publishing groups of like compositions, usually in sets of three-, six-, and twelve compositions. Consequently, opus numbers usually are not chronologic, unpublished compositions usually had no opus number, and numeration gaps and sequential duplications occurred when publishers issued contemporaneous editions of a composer’s works, as in the sets of string quartets by Joseph Haydn (1732–1809) and Ludwig van Beethoven (1770–1827); Haydn’s Op. 76, the Erdödy quartets (1796–97), comprises six discrete quartets consecutively numbered Op. 76 No. 1 – Op. 76 No. 6; whilst Beethoven’s Op. 59, the Rasumovsky quartets (1805–06), comprises String Quartet No. 7, String Quartet No. 8, and String Quartet No. 9.
19th century to date
From about 1800, composers, especially Ludwig van Beethoven, assigned an opus number to a work, and later to a set of works, especially songs and short piano pieces; however, composers’ inconsistent usages ended the correspondence between an opus number and the work’s publication date. Since approximately 1900, composers tended to assign an opus number to a composition, published or not. Early in his career, Beethoven selectively enumerated his compositions (some published without opus numbers), yet in later years, he published early works with high opus numbers. Likewise, some posthumously published works were given high opus numbers by publishers, even though some of them were written early in Beethoven's career. Since his death in 1827, the un-numbered compositions have been catalogued and labelled with the German acronym WoO (Werk ohne Opuszahl), meaning "work without opus number". However, there are other catalogues of Beethoven's works - see Catalogues of Beethoven compositions.
Examples of composers’ historically-inconsistent opus number usages include the cases of César Franck (1822–1890) and Béla Bartók (1881–1945) who initially enumerated, but then discontinued enumerating, their composistions. Carl Nielsen (1865–1931) and Paul Hindemith (1895–1963) were also inconsistent in their approaches. Moreover, Sergei Prokofiev (1891–1953) was a consistent enumerator who assigned an opus number to a composition before composing it; at his death he left fragmentary and planned — but numbered — works. In revising a composition, Prokofiev occasionally assigned a new opus number to the revision, thus: Symphony No. 4 is two, thematically-related, but discrete works: (i) Symphony No. 4, Op. 47, and (ii) Symphony No. 4, Op. 112; the former, Op. 47, was written in 1929; the latter, Op. 112, is a large-scale revision written in 1947. Like-wise, depending upon the edition, the original version of Piano Sonata No. 5 in C major, is twice catalogued as: (i) Piano Sonata No. 5, Op. 38, and as (ii) Piano Sonata No. 5, Op. 135.
To supersede composers’ inconsistent opus number usage, especially by those of the baroque and classical eras, musicologists have definitively and unambiguously catalogued them via thematic catalogue abbreviations.
The works of Carl Friedrich Abel, usually catalogued by original publication opus numbers, e.g. the Op. 17 symphonies have different catalogue opus numbers Walter Knape assigned them in the Bibliographisch-thematisches Verzeichnis der Kompositionen von Karl Friedrich Abel (Bibliographic–Thematic Catalogue of the Compositions of Karl Friedrich Abel), published by Cuxhaven, in 1972.
Carl Philipp Emanuel Bach's works are indexed with two opus number systems: (i) the older Wotquenne (Wq.) numbering system, established by Alfred Wotquenne, in a catalogue of Emanuel’s music, in 1905; and (ii) the complete H. opus number system by E. Eugene Helm, used in the Thematic Catalogue of the Works of C.P.E. Bach (New Haven: Yale University Press, 1989).
Johann Christian Bach's works are mostly referred to by the original-publisher’s assigned opus number, which can cause mis-identifications, because different, secondary publishers used the same (original) opus number, such as using Op. 18 to identify three different sets of J. C. Bach works: (i) “Six Grand Overtures”, (ii) “Deux sinfonies”, and (iii) “Four Sonatas and Two Duets”. Moreover, three of six Op. 6 symphonies are identified as Op. 8, but in a different order. Because of said confusion, the de facto standard catalogue is John Christian Bach, by C. S. Terry (2nd ed. 1967), for identifying a J. C. Bach work, one cites the page and incipit numbers “in Terry”, although they are not catalogue numbers. Moreover, a short list of the Terry numbers is in The New Grove Bach Family, by Christoph Wolff, et al. (pp. 341ff., NY: Norton, 1983). Also used are the opus numbers of the “Thematic Catalog” in the Collected Works of Johann Christian Bach (Ernest Warburton, ed.; NY: Garland Publishing, 1985).
Antonín Dvořák's works are usually now referenced by B numbers, after Jarmil Burghauser's comprehensive catalogue which resolved a great many difficulties with the often misleading and duplicated opus numbers given by different publishers to Dvořák's works.
César Franck's works are usually now referred to by M numbers or FWV numbers, although he did allocate opus numbers to some early compositions. M and FWV refer to the catalogue created by Wilhelm Mohr, which he himself called Franck-Werke-Verzeichnis
Joseph Haydn's works are referred to by their Hob or Hoboken numbers after Anthony van Hoboken's 1957 classification. Hoboken assigned numbers to the string quartets, but these are generally still known by their opus numbers.
Franz Liszt's works are referred to by their S or Searle numbers after Humphrey Searle's 1960s classification The Music of Liszt. Alternately, R is used to refer to Peter Raabe's 1931 reference Franz Liszt: Leben und Schaffen.
Antonio Rosetti's works are usually given with catalog numbers by Sterling E. Murray, Chairman of the Department of Music History at West Chester University of Pennsylvania, although older numbers from Oskar Kaul's 1912 Rosetti catalog sometimes appear as well. For example, Rosetti's popular "La Chasse" symphony is numbered as "Murray A20/Kaul I:18."
Franz Schubert's works are referred to by their D or Deutsch numbers after Otto Erich Deutsch's catalogue. Schubert's opus numbers are very scattered, unchronological, and mostly posthumous, but a few of them are occasionally seen.
Antonio Soler's keyboard sonatas are usually referred to by their R number, after the catalogue compiled by Father Samuel Rubio.
Antonio Vivaldi's works are referred to by their RV or Ryom-Verzeichnis numbers after Peter Ryom's catalogue. Some of his works were published in opus sets, and these numbers are often still used as well.
Richard Wagner's works are referred to by their WWV or Wagner-Werke-Verzeichnis numbers, which also include his non-musical work.
Carl Maria von Weber's works may appear by opus or by J. number, the latter referring to Friedrich Wilhelm Jähns, Carl Maria von Weber in Seinen Werken: Chronologisch-Thematisches Verzeichnis Seiner Sämmtlichen Compositionen (1871). A list of Weber's works in order of Jähns catalogue number can be found at List of compositions by Carl Maria von Weber.
In a parody of catalogue numbering, the works of P. D. Q. Bach are assigned "Schickele" numbers, after Peter Schickele, the works' sole discoverer (and, in reality, their composer). Schickele numbers are not sequential but are intended as jokes (a Christmas work is S.359 because 25 December is the 359th day of the (non-leap) year.